/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.util.concurrent;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtIncompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

/**
 * An object with an operational state, plus asynchronous {@link #startAsync()} and
 * {@link #stopAsync()} lifecycle methods to transition between states. Example services include
 * webservers, RPC servers and timers.
 *
 * <p>
 * The normal lifecycle of a service is:
 * <ul>
 * <li>{@linkplain State#NEW NEW} -&gt;
 * <li>{@linkplain State#STARTING STARTING} -&gt;
 * <li>{@linkplain State#RUNNING RUNNING} -&gt;
 * <li>{@linkplain State#STOPPING STOPPING} -&gt;
 * <li>{@linkplain State#TERMINATED TERMINATED}
 * </ul>
 *
 * <p>
 * There are deviations from this if there are failures or if {@link Service#stopAsync} is called
 * before the {@link Service} reaches the {@linkplain State#RUNNING RUNNING} state. The set of legal
 * transitions form a <a href="http://en.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a>,
 * therefore every method of the listener will be called at most once. N.B. The {@link State#FAILED}
 * and {@link State#TERMINATED} states are terminal states, once a service enters either of these
 * states it cannot ever leave them.
 *
 * <p>
 * Implementors of this interface are strongly encouraged to extend one of the abstract classes in
 * this package which implement this interface and make the threading and state management easier.
 *
 * @author Jesse Wilson
 * @author Luke Sandberg
 * @since 9.0 (in 1.0 as {@code com.google.common.base.Service})
 */
@Beta
@GwtIncompatible
public interface Service {
    /**
     * If the service state is {@link State#NEW}, this initiates service startup and returns
     * immediately. A stopped service may not be restarted.
     *
     * @return this
     * @throws IllegalStateException if the service is not {@link State#NEW}
     *
     * @since 15.0
     */
    @CanIgnoreReturnValue
    Service startAsync();

    /**
     * Returns {@code true} if this service is {@linkplain State#RUNNING running}.
     */
    boolean isRunning();

    /**
     * Returns the lifecycle state of the service.
     */
    State state();

    /**
     * If the service is {@linkplain State#STARTING starting} or {@linkplain State#RUNNING running},
     * this initiates service shutdown and returns immediately. If the service is
     * {@linkplain State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having
     * been started nor stopped. If the service has already been stopped, this method returns
     * immediately without taking action.
     *
     * @return this
     * @since 15.0
     */
    @CanIgnoreReturnValue
    Service stopAsync();

    /**
     * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state}.
     *
     * @throws IllegalStateException if the service reaches a state from which it is not possible to
     *         enter the {@link State#RUNNING} state. e.g. if the {@code state} is
     *         {@code State#TERMINATED} when this method is called then this will throw an
     *         IllegalStateException.
     *
     * @since 15.0
     */
    void awaitRunning();

    /**
     * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state} for no
     * more than the given time.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @throws TimeoutException if the service has not reached the given state within the deadline
     * @throws IllegalStateException if the service reaches a state from which it is not possible to
     *         enter the {@link State#RUNNING RUNNING} state. e.g. if the {@code state} is
     *         {@code State#TERMINATED} when this method is called then this will throw an
     *         IllegalStateException.
     *
     * @since 15.0
     */
    void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException;

    /**
     * Waits for the {@link Service} to reach the {@linkplain State#TERMINATED terminated state}.
     *
     * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
     *
     * @since 15.0
     */
    void awaitTerminated();

    /**
     * Waits for the {@link Service} to reach a terminal state (either
     * {@link Service.State#TERMINATED terminated} or {@link Service.State#FAILED failed}) for no
     * more than the given time.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @throws TimeoutException if the service has not reached the given state within the deadline
     * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
     * @since 15.0
     */
    void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException;

    /**
     * Returns the {@link Throwable} that caused this service to fail.
     *
     * @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
     *
     * @since 14.0
     */
    Throwable failureCause();

    /**
     * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
     * executor. The listener will have the corresponding transition method called whenever the
     * service changes state. The listener will not have previous state changes replayed, so it is
     * suggested that listeners are added before the service starts.
     *
     * <p>
     * {@code addListener} guarantees execution ordering across calls to a given listener but not
     * across calls to multiple listeners. Specifically, a given listener will have its callbacks
     * invoked in the same order as the underlying service enters those states. Additionally, at
     * most one of the listener's callbacks will execute at once. However, multiple listeners'
     * callbacks may execute concurrently, and listeners may execute in an order different from the
     * one in which they were registered.
     *
     * <p>
     * RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown during
     * {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
     * logged.
     *
     * @param listener the listener to run when the service changes state is complete
     * @param executor the executor in which the listeners callback methods will be run. For fast,
     *        lightweight listeners that would be safe to execute in any thread, consider
     *        {@link MoreExecutors#directExecutor}.
     * @since 13.0
     */
    void addListener(Listener listener, Executor executor);

    /**
     * The lifecycle states of a service.
     *
     * <p>
     * The ordering of the {@link State} enum is defined such that if there is a state transition
     * from {@code A -> B} then {@code A.compareTo(B) < 0}. N.B. The converse is not true, i.e. if
     * {@code A.compareTo(B) < 0} then there is <b>not</b> guaranteed to be a valid state transition
     * {@code A -> B}.
     *
     * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
     */
    @Beta // should come out of Beta when Service does
    enum State {
        /**
         * A service in this state is inactive. It does minimal work and consumes minimal resources.
         */
        NEW {
            @Override
            boolean isTerminal() {
                return false;
            }
        },

        /**
         * A service in this state is transitioning to {@link #RUNNING}.
         */
        STARTING {
            @Override
            boolean isTerminal() {
                return false;
            }
        },

        /**
         * A service in this state is operational.
         */
        RUNNING {
            @Override
            boolean isTerminal() {
                return false;
            }
        },

        /**
         * A service in this state is transitioning to {@link #TERMINATED}.
         */
        STOPPING {
            @Override
            boolean isTerminal() {
                return false;
            }
        },

        /**
         * A service in this state has completed execution normally. It does minimal work and
         * consumes minimal resources.
         */
        TERMINATED {
            @Override
            boolean isTerminal() {
                return true;
            }
        },

        /**
         * A service in this state has encountered a problem and may not be operational. It cannot
         * be started nor stopped.
         */
        FAILED {
            @Override
            boolean isTerminal() {
                return true;
            }
        };

        /** Returns true if this state is terminal. */
        abstract boolean isTerminal();
    }

    /**
     * A listener for the various state changes that a {@link Service} goes through in its
     * lifecycle.
     *
     * <p>
     * All methods are no-ops by default, implementors should override the ones they care about.
     *
     * @author Luke Sandberg
     * @since 15.0 (present as an interface in 13.0)
     */
    @Beta // should come out of Beta when Service does
    abstract class Listener {
        /**
         * Called when the service transitions from {@linkplain State#NEW NEW} to
         * {@linkplain State#STARTING STARTING}. This occurs when {@link Service#startAsync} is
         * called the first time.
         */
        public void starting() {}

        /**
         * Called when the service transitions from {@linkplain State#STARTING STARTING} to
         * {@linkplain State#RUNNING RUNNING}. This occurs when a service has successfully started.
         */
        public void running() {}

        /**
         * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state.
         * The only valid values for {@code from} are {@linkplain State#STARTING STARTING} or
         * {@linkplain State#RUNNING RUNNING}. This occurs when {@link Service#stopAsync} is called.
         *
         * @param from The previous state that is being transitioned from.
         */
        public void stopping(State from) {}

        /**
         * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED}
         * state. The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the
         * transition diagram. Therefore, if this method is called, no other methods will be called
         * on the {@link Listener}.
         *
         * @param from The previous state that is being transitioned from. The only valid values for
         *        this are {@linkplain State#NEW NEW}, {@linkplain State#RUNNING RUNNING} or
         *        {@linkplain State#STOPPING STOPPING}.
         */
        public void terminated(State from) {}

        /**
         * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The
         * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.
         * Therefore, if this method is called, no other methods will be called on the
         * {@link Listener}.
         *
         * @param from The previous state that is being transitioned from. Failure can occur in any
         *        state with the exception of {@linkplain State#NEW NEW} or
         *        {@linkplain State#TERMINATED TERMINATED}.
         * @param failure The exception that caused the failure.
         */
        public void failed(State from, Throwable failure) {}
    }
}
